CETERACH. 
151 
Genus VIII. CETEHACH, or SCALE PEIllSr. 
Of the genus Ceterach there is only one British species, 
and this is so different from all others as to be distinctly 
recognized at a glance. The mark by which it is known 
is this : — the back of every frond is covered by densely- 
packed, brown, pointed, chaffy scales. Among these 
scales, and concealed by them, lie the elongate sori, which 
are anomalous, in regard to their relationship, in having 
no indusium. The affinity of Ceterach is without doubt 
with the Asplenium-X^k^ Ferns ; and this being the case, 
they ought to have an indusium ; the PolypodiumAXkQ and 
Acrostichum-]XkQ Ferns chiefly, among the dorsal groups, 
wanting this cover to the sori. No indusium, however, 
exists here, unless it be represented by a kind of mem- 
branous ridge, which is to be found on the receptacles just 
behind the sori, and is the part which has been sometimes 
called an indusium. The probability is, that it does 
represent that organ, which is not largely developed in 
consequence of the presence of so dense a covering of scales, 
these not only serving the purpose of a cover to the sori, 
